This invention relates to a process for improving the rust resistance of thinly coated tinplate covered with 1.5 g/m.sup.2 or less of thin for the manufacture of cans for foods and drinks, the seams of such can bodies being joined by welding.
Solder has been used to join the seams of can bodies up to the present when cans for foods and drinks are manufactured. However, because poisonous lead contained in solder may dissolve in the contents of cans, solder made of pure tin has come to be used to join the seams of can bodies recently. In addition, seam welding (Soudronic welding) in place of soldering using expensive tin is being popularized to join the seams. Since the width of (overlapped) seams of can bodies in the Soudronic welding is 0.3-0.4 mm, the quality of welds is considerably affected by the method of treating the surface of a steel sheet. The smaller the plated amount of tin in tinplace on the market, the better its weldability tends to become. One of the reasons for this is that, for tinplate coated with the relatively large amount of tin, heat generated in resistance welding is also used to fuse tin, which flows along the weld line and causes the pickup of the weld zone. When thinly coated tinplate is used as material for making cans and particularly when cans with a coated inside surface are used, the material desired is, since the tin price is rising, the one which is coated with less than half of the amount of tin covering #25 tinplate, so to speak, which is said to be coated with the smallest amount of tin (2.8 g/m.sup.2) on the market. In the case of this type of tinplate coated with the small amount of tin, however, it may generate rust during a period of several months after it has been manufactured and shipped until it is used for can manufacturing on the part of users, and in some circumstances may become unusable for can manufacturing.